JP7824752B2 - Semiconductor optical element and its manufacturing method - Google Patents

Description

The present invention relates to a semiconductor optical device and a method for manufacturing the same.

In today's internet-based society, demand for high-speed, high-capacity semiconductor optical devices is increasing. Known examples of semiconductor optical devices include direct modulation semiconductor lasers and semiconductor lasers that emit continuous light. External modulators that modulate continuous light to generate optical signals include electroabsorption modulators and Mach-Zehnder modulators. The basic structure of these semiconductor optical devices generally consists of a multi-quantum well (MQW) layer sandwiched between p-type and n-type semiconductors. The MQW functions as a light-generating layer when the semiconductor optical device is a semiconductor laser and as an absorption layer when the semiconductor optical device is a modulator. Furthermore, a structure in which the MQW is sandwiched between optical confinement layers (SCH layers; Separate Confinement Heterostructure layers) is also known. The SCH layer is arranged, for example, to confine light to the MQW with high efficiency. For this reason, the refractive index of the SCH layer is generally smaller than the refractive index of the well layers and barrier layers that form the MQW. In other words, the compositional wavelength of the SCH layer is generally shorter than the compositional wavelength of the well layers and barrier layers that form the MQW. The fact that the compositional wavelength of the SCH layer is shorter than that of the MQW indicates a larger band gap, and for example, under forward bias, carriers can move smoothly from the SCH layer toward the MQW.

Patent Document 1 discloses a structure in which two n-type SCH layers are provided, with the composition wavelength of the second confinement layer closer to the semiconductor substrate being longer than that of the barrier layer. This structure increases the carrier capture time, thereby increasing frequency modulation efficiency.

Japanese Patent Application Publication No. 07-183617

The characteristics of semiconductor optical devices depend heavily on the quality of the MQW. MQWs are fabricated using crystal growth techniques such as MOCVD (Metal Organic Chemical Vapor Deposition). When forming an SCH layer, the upper and lower SCH layers are often grown simultaneously (continuously) to sandwich the MQW. PL measurement (photoluminescence) is a well-known method for evaluating the quality of the MQW. PL measurement is a semiconductor evaluation technique that involves irradiating a semiconductor with external excitation light and measuring the spontaneous emission of carriers generated. A light source that emits light with a wavelength absorbed by the semiconductor layer being evaluated is used as the excitation light source. Spontaneous emission from the MQW can be measured by using an excitation light source with a wavelength that can be absorbed only by the MQW. However, because the compositional wavelengths of the MQW and SCH layers are often close, a light source that emits light with a wavelength that can be absorbed by both the MQW and the SCH layer is often used. The spontaneous emission spectrum measured here includes both the spontaneous emission from the MQW and the spontaneous emission from the SCH layer.

Crystal quality can be inspected by analyzing the spontaneous emission spectrum. However, for example, if the spontaneous emission intensity is low, the spectrum cannot be analyzed accurately. Furthermore, during mass production, if the spontaneous emission spectrum intensity varies significantly between lots, it can be difficult to determine which spectrum is correct. There are several reasons for the low spontaneous emission intensity. One is that the MQW is below the desired crystal quality, resulting in low spontaneous emission and insufficient absorption of pump light. Another is that the MQW itself has a structure that makes it difficult to absorb pump light (for example, a thin layer thickness), resulting in low absorption of pump light. Furthermore, while the intensity of the pump light is generally set to ensure sufficient spontaneous emission intensity from the MQW, measurement variations and other factors can sometimes prevent the desired pump light from being irradiated onto the semiconductor device, resulting in reduced spontaneous emission intensity from the MQW. Therefore, if the intensity of the evaluation spectrum in PL measurements is low, it may not be possible to determine whether the crystal quality of the MQW is poor, or whether the MQW simply has a structure that does not allow it to sufficiently absorb the excitation light, or whether the MQW was not sufficiently irradiated with the excitation light, and it may not be possible to evaluate the crystal quality of the MQW.

When evaluating semiconductor layers compatible with the 1.3 μm band used in optical communications, a YAG laser with an oscillation wavelength of 1064 nm is often used as the excitation light source. InP-based semiconductor optical devices often use InP as the substrate. The energy of the excitation light from a YAG laser is smaller than the band gap of InP. Therefore, when MQWs compatible with the 1.3 μm band are grown on an InP substrate, the YAG laser excites the MQWs without exciting the InP substrate, making it suitable for evaluating the grown layer alone.

Here, the composition wavelength of the SCH layer of a 1.3 μm-band semiconductor optical device may be set to approximately 1.00 μm to 1.1 μm. However, during crystal growth, the composition wavelength of the SCH layer is affected by manufacturing variations. For example, even if the SCH layer's set wavelength is designed to be 1.05 μm, the actual composition wavelength of the SCH layer may be 1.03 μm or 1.07 μm. In this case, when 1064 nm (1.064 μm) pump light is incident, the pump light may or may not be absorbed by the SCH layer. If the pump light is not absorbed by the SCH layer, it is absorbed only by the MQW, allowing the spontaneous emission from the MQW to be measured, thereby confirming the quality of the MQW. However, if the pump light is absorbed by the SCH layer, the measured spectrum is a spontaneous emission spectrum that includes light emitted not only from the MQW but also from the SCH layer. Furthermore, because light is absorbed not only by the MQW but also by the SCH layer, more carriers are excited, and the intensity of spontaneously emitted light from the MQW changes compared to when light is absorbed only by the MQW. If the SCH layer always absorbs light, it would be possible to compare the quality of stacked semiconductor layers, taking into account that light is absorbed by the SCH layer. However, because the composition wavelength of the SCH layer varies due to manufacturing variations, if there is a mixture of cases in which the SCH layer absorbs light and cases in which it does not between lots, the results of PL measurements will vary, making it difficult to accurately confirm the quality of the semiconductor multilayer.

In response to the above-mentioned issues, the present invention aims to provide a semiconductor optical device with a structure that allows evaluation of the crystal quality of semiconductor layers, particularly multi-quantum well layers, and that is suitable for mass production.

(1) A semiconductor optical device according to the present disclosure comprises a substrate of a first conductivity type; an optical confinement layer of a first conductivity type disposed above the substrate of the first conductivity type; a multiple quantum well layer composed of a plurality of well layers and barrier layers disposed above the optical confinement layer of the first conductivity type; an optical confinement layer of a second conductivity type disposed on the multiple quantum well layer; and a PL stabilizing layer disposed between the substrate of the first conductivity type and the multiple quantum well layer, wherein the thickness of the PL stabilizing layer is at least half the thickness of the multiple quantum well layer, and the composition wavelength of the PL stabilizing layer is shorter than the composition wavelength of the well layers of the multiple quantum well layer and longer than the composition wavelength of the optical confinement layer of the first conductivity type.

This invention makes it possible to stably inspect the crystal quality of the multiple quantum well layers of semiconductor optical devices, thereby achieving stable mass production.

1 is a top view of a semiconductor optical device according to a first embodiment of the present invention. 2 is a cross-sectional view taken along line AA in FIG. 1. 3A to 3C are cross-sectional views illustrating a manufacturing process of the semiconductor optical device according to the first embodiment. FIG. 4 is a top view of a semiconductor optical device according to a second embodiment of the present invention. 5 is a cross-sectional view taken along line AA in FIG. 4. FIG. 6 is an enlarged view of the cross section taken along line AA in FIG. 5. 10A to 10C are cross-sectional views illustrating a semiconductor laser according to a second embodiment during manufacturing. FIG. 10 is a schematic diagram of a band diagram of a semiconductor laser according to a second embodiment. FIG. 10 is a cross-sectional view of a semiconductor optical device according to a third embodiment of the present invention taken along line AA. FIG. 10 is a cross-sectional view taken along line AA of a semiconductor optical device according to a fourth embodiment of the present invention.

Embodiments of the present invention will be described in detail below with reference to the drawings. Note that in all figures used to explain the embodiments, components having the same functions will be given the same reference numerals, and repeated explanations will be omitted. Note that the figures shown below are merely for explaining examples of the embodiments, and the size of the figures does not necessarily correspond to the scale described in these examples.

[First embodiment]
FIG. 1 is a top view of a semiconductor optical device 1 according to a first embodiment of the present invention. The semiconductor optical device 1 is a semiconductor laser. FIG. 2 is a schematic diagram of the A-A cross section perpendicular to the optical axis. The semiconductor optical device 1 includes a structure in which a PL stabilization layer 9, an n-type optical confinement layer (n-type SCH layer 3), a multiple quantum well (MQW) layer 4, a p-type SCH layer 5, and a p-type cladding layer 6 are stacked in this order on an n-type substrate (substrate 2). The semiconductor optical device 1 has a front electrode 7 and a back electrode 8, with the front electrode 7 in contact with the p-type cladding layer 6 and the back electrode 8 in contact with the n-type (substrate 2). The MQW 4 emits light by applying a voltage (injecting a current) between these two electrodes. Note that the n-type and p-type may be reversed. Furthermore, a contact layer may be disposed between the p-type cladding layer 6 and the front electrode 7 to reduce electrical resistance.

Here, the PL stabilization layer 9 is a layer with a longer composition wavelength (smaller band gap) than the n-type SCH layer 3. Furthermore, the PL stabilization layer 9 has a composition wavelength that can absorb the excitation light of the excitation light source described below.

This disclosure is characterized by the PL stabilization layer 9. To explain the effects of the PL stabilization layer 9, the manufacturing procedure for the semiconductor optical device 1 will be described. First, as shown in Figure 3, the PL stabilization layer 9, n-type SCH layer 3, MQW 4, p-type SCH layer 5, and cap layer 10 are crystal-grown over the entire top surface of the substrate 2. After crystal growth, PL measurements are performed to confirm the crystal quality of the MQW 4. PL measurements are performed by irradiating excitation light of a wavelength absorbable by the MQW 4 and PL stabilization layer 9 onto the surface of the cap layer 10 and measuring spontaneous emission (PL) light from the semiconductor optical device 1. While excitation light can be irradiated from the substrate 2 side, it is preferable to irradiate it from the cap layer 10 side for stable evaluation because the substrate 2 is sufficiently thick compared to the other layers, and the excitation light may be absorbed or scattered by the substrate 2. Note that the cap layer 10 is provided to protect the layers below the p-type SCH layer 5 during other manufacturing processes, such as PL measurements, and does not ultimately remain. Therefore, the cap layer 10 is optional.

In PL measurements, a wavelength spectrum corresponding to the band gap of each layer is measured. PL measurements are a method for confirming crystal quality by evaluating the emission intensity, half-width, etc. of this wavelength spectrum. To confirm the crystal quality of MQW4, the wavelength of the excitation light must be shorter than the composition wavelength of the well layers that make up the MQW4. In this case, the well layers may be too thin to adequately absorb the excitation light. Furthermore, if the composition wavelengths of the n-type SCH3 and p-type SCH5 are shorter than the wavelength of the excitation light, the two SCH layers may not absorb the excitation light. As a result, the intensity of the PL light output from the MQW4 is low, making accurate PL measurements impossible. However, the PL stabilization layer 9 in this disclosure enables stable PL measurements. The PL stabilization layer 9 is formed with a composition wavelength that can absorb the excitation light. Therefore, the excitation light that is not absorbed by the MQW4 (excitation light that passes through the MQW4) is absorbed by the PL stabilization layer 9. The excitation light absorbed by the PL stabilization layer 9 generates electrons and holes (carriers). Some of the generated carriers migrate to the MQW4 due to energy such as heat. The migrated carriers recombine in the MQW4, generating PL light. This increases the intensity of the PL light from the MQW4, enabling accurate PL measurements. The compositional wavelength of the PL stabilization layer 9 is shorter than the compositional wavelength of the well layers of the MQW4. If the compositional wavelength of the PL stabilization layer 9 is longer than that of the well layers, the carriers excited in the PL stabilization layer 9 recombine there, and the PL light from the semiconductor optical device 1 is mainly emitted from the PL stabilization layer 9. This reduces the PL light from the MQW4, making it impossible to evaluate the crystallinity of the MQW4. Furthermore, it is desirable for the compositional wavelength of the PL stabilization layer 9 to be longer than that of the n-type SCH layer 3. If the compositional wavelength of the n-type SCH layer 3 is longer than that of the PL stabilization layer 9, the excitation light can be absorbed by the n-type SCH layer 3, allowing stable PL measurements even without the PL stabilization layer 9. In this case, the PL stabilization layer 9 is not necessary. However, as will be described later, the PL stabilization layer 9 may be effective even if the composition wavelength of the n-type SCH layer 3 is such that it can absorb the excitation light.

Here, the thickness of the PL stabilization layer 9 is preferably at least half the thickness of the MQW4. Furthermore, by making the thickness at least twice the total thickness of the layers capable of absorbing excitation light, excluding the MQW4, stable PL measurements can be achieved. For example, if the n-type SCH layer 3 and the p-type SCH layer 5 (hereinafter, the n-type SCH layer 3 and the p-type SCH layer 5 will be collectively referred to as the SCH layer) also have composition wavelengths that can absorb excitation light, carriers generated in the SCH layer migrate to the MQW4 and are output as PL light. However, if the SCH layer is thin, the number of carriers supplied is small, and the PL light intensity remains low. However, if the PL stabilization layer 9 is arranged with a thickness at least twice the total thickness of the n-type SCH layer 3 and the p-type SCH layer 5, a sufficient supply of carriers can be achieved.

Furthermore, manufacturing variations can cause the composition wavelength of the SCH layer to deviate from the design value. For example, when the composition wavelength of the SCH layer and the wavelength of the pump light are close to each other, manufacturing variations can cause the composition wavelength of the SCH layer to either absorb the pump light or not. Whether or not the SCH layer absorbs the pump light changes the amount of carriers supplied to the MQW, which in turn changes the PL light intensity from the MQW. However, if the PL stabilization layer 9 is provided, carriers can be stably supplied to the MQW regardless of whether or not the SCH layer absorbs the pump light. In a structure including the PL stabilization layer 9, the intensity of the PL light generated in the MQW can change depending on the presence or absence of pump light in the SCH layer, but the spectral shape of the PL light generated in the MQW is stable, allowing for stable PL measurements.

Second Embodiment
4 is a top view of a semiconductor optical device 201 according to a second embodiment of the present disclosure. The semiconductor optical device 201 is a modulator-integrated semiconductor laser in which a semiconductor laser 211, an electro-absorption modulator (EA modulator) 213, and a waveguide 212 are integrated. While this embodiment illustrates an example of an integrated type, the present disclosure can also achieve the effects of a semiconductor laser alone or an EA modulator alone. The semiconductor optical device 201 is an integrated device in which the semiconductor laser 211, the waveguide 212, and the EA modulator 213 are optically connected to each other in this order. The semiconductor laser 211 emits continuous light, and the waveguide 212 transmits the light emitted from the semiconductor laser 211 to the EA modulator 213. The EA modulator 213 has a multiple quantum well layer that absorbs light corresponding to the oscillation wavelength of the semiconductor laser 211. The continuous light that passes through the waveguide 212 and enters the EA modulator 213 is intensity-modulated by the EA modulator 213 and converted into a modulated optical signal, such as a binary or quaternary modulated signal. The modulated optical signal output from the EA modulator 213 is output from a front end facet 221. Note that another structure, such as a window structure using semi-insulating InP, may be provided near the front end facet 221. A dielectric non-reflective coating (not shown) is formed on the front end facet 221. Furthermore, a dielectric high-reflective coating (not shown) is formed on a rear end facet 222, which is the end face on the opposite side of the semiconductor laser 211. As will be described in detail later, the semiconductor optical device 1 has a mesa structure from the semiconductor laser 211 to the EA modulator 213, and both sides of the mesa structure are buried with semiconductor burying layers (BH layers 215), forming a buried semiconductor device.

FIG. 5 is a schematic diagram of the AA cross section perpendicular to the optical axis of the semiconductor laser 211. The mesa structure is composed of a portion of the substrate 202 (n-InP substrate), the laser portion multiple quantum well layer 204, the p-type cladding layer 216, and the p-type contact layer 218 shown in FIG. 6. In this example, the p-type cladding layer 216 is composed of a p-InP layer. Both sides of the mesa structure are buried with buried layers (BH layers 215). Details of the mesa structure will be described later. An insulating film 219 is disposed on the upper surface of the BH layer 215, with the exception of a portion. The insulating film 219 is, for example, a SiO ₂ film. A laser portion electrode 207 is disposed across a portion of the upper surface of the insulating film 219 and the top of the mesa structure. A backside electrode 208 is disposed on the backside of the substrate 202. The semiconductor laser 211 generates continuous light by applying a voltage (injecting a current) between the laser portion electrode 207 and the backside electrode 208.

Figure 6 is an enlarged view of the mesa structure and its vicinity of the semiconductor laser 211 shown in Figure 5. The mesa structure is composed of a portion of the substrate 202, a PL stabilization layer 209, an n-type optical confinement layer (SCH layer) 203, a laser portion multiple quantum well layer 204, a p-type SCH layer 205, a p-type cladding layer 216, and a p-type contact layer 218. A diffraction grating is formed in the p-type SCH layer 205, and the semiconductor laser 211 is a DFB (Distributed Feedback) laser that oscillates in the 1.3 μm band. Furthermore, the semiconductor laser 211 is not limited to a DFB laser; it may also be a FP (Fabry-Perot) laser, a DBR (Distributed Bragg Reflector) laser, or a DR (Distributed Reflector) laser.

The PL stabilization layer 209 is composed of an undoped InGaAsP layer with a thickness of 100 nm and a composition wavelength of 1.1 μm. The n-type SCH layer 203 is composed of n-type InGaAsP with a thickness of 25 nm and a composition wavelength of 1.05 μm. The p-type SCH layer 205 is composed of p-type InGaAsP with a thickness of 25 nm and a composition wavelength of 1.05 μm. The laser portion multiple quantum well layer 204 is composed of multiple barrier layers and well layers arranged alternately. Note that the laser portion multiple quantum well layer 204 begins and ends with a barrier layer. Here, both the well layer and barrier layer are 8 nm thick. The barrier layer is undoped InGaAsP with a composition wavelength of 1.08 μm, and the well layer is undoped InGaAsP with a composition wavelength of 1.28 μm. The entire laser portion multiple quantum well layer 204 has a composition compatible with the 1.3 μm band. The p-type cladding layer 216 is a p-type InP layer with a thickness of 1500 nm. Note that the composition wavelength and thickness described here are merely examples given to provide a detailed explanation of the structure that will provide the benefits of the present disclosure; the benefits of the present disclosure can also be achieved with different composition wavelengths and layer thicknesses.

This disclosure is characterized by the PL stabilization layer 209. To explain the effects of the PL stabilization layer 209, the manufacturing procedure for the semiconductor optical device 201, particularly the manufacturing procedure for the semiconductor laser 211, will be described. First, as shown in FIG. 7, the PL stabilization layer 209, n-type SCH layer 203, laser portion multi-quantum well layer 204, p-type SCH layer 205, and p-type InP cap layer 210 are crystal-grown on the entire upper surface of the substrate 202. Note that an n-InP buffer layer may be disposed between the substrate 202 and the PL stabilization layer 209. After crystal growth, PL measurements are performed to confirm the crystal quality of the laser portion multi-quantum well layer 204. PL measurements are performed by irradiating the surface side of the p-type InP cap layer 210 with a YAG laser having an oscillation wavelength of 1064 nm and measuring the light emission (PL light) from the semiconductor laser 211.

The output light (pump light) of the YAG laser is 1064 nm (1.064 μm), so it is absorbed by layers with compositional wavelengths longer than this wavelength and is transmitted through layers with compositional wavelengths shorter than 1.064 μm. In this configuration, the pump light is absorbed by the well layer and PL stabilization layer 209.

Figure 8 shows a band diagram that schematically illustrates the optical absorption of the semiconductor laser 211 when a YAG laser beam is incident. The pump light is absorbed in the well layer, generating electron-hole pairs. The electrons and holes recombine to output PL light with a wavelength spectrum corresponding to the band gap of the laser portion multiple quantum well layer 204. Not all of the pump light is absorbed by the well layer; some reaches the substrate 202. At this time, the pump light is also absorbed by the PL stabilization layer 209. Electrons and holes (carriers) are generated by the pump light absorbed by the PL stabilization layer 209. Some of the generated carriers migrate toward the laser portion multiple quantum well layer 204 due to energy such as heat. The migrated carriers recombine in the laser portion multiple quantum well layer 204, outputting PL light. The PL stabilization layer 209 is 100 nm thick, so it sufficiently absorbs the pump light, providing the laser portion multiple quantum well layer 204 with a sufficient supply of carriers for PL emission. As a result, the PL light output from the laser portion multiple quantum well layer 204 has sufficient intensity to confirm its quality.

As noted in the problem section, the SCH layer of a 1.3 μm-band semiconductor optical device is often close to the wavelength of a YAG laser. In this embodiment, both the n-type SCH layer 203 and the p-type SCH layer 205 are 1.05 μm. However, even if the designed composition wavelength is 1.05 μm, manufacturing variations of approximately ±0.03 μm from the designed wavelength occur. If the variation is toward the longer wavelength side, the composition wavelength of the SCH layer may become a composition wavelength that absorbs light with a wavelength of 1.064 μm. Conversely, if the composition wavelength of the SCH layer varies toward the shorter wavelength side, it will deviate from 1.064 μm. As a result, variations occur in the amount of pump light absorbed. If the composition wavelength of the SCH layer shifts toward the longer wavelength side, the pump light is absorbed, carriers are supplied to the laser portion multiple quantum well layer 204, and the PL light intensity increases. On the other hand, if the composition wavelength of the SCH layer is shifted to the shorter wavelength side, the pump light is not absorbed, but is absorbed only by the well layer, so the PL intensity may be lower than when the SCH layer is shifted to the longer wavelength side. In other words, when multiple semiconductor optical devices with the same structure are manufactured, the PL intensity will vary depending on the composition wavelength of the finished SCH layer, making it difficult to determine which one represents the correct PL measurement. This makes stable mass production difficult.

On the other hand, as described above, by making the PL stabilization layer 209 sufficiently thick compared to the other layers and by selecting a composition wavelength that reliably absorbs YAG laser light, the PL stabilization layer 209 can reliably absorb the excitation light and supply carriers to the laser portion multi-quantum well layer 204. A certain amount of electrons is supplied, and the intensity of the PL light is stable. To ensure stable light absorption, the PL stabilization layer 209 preferably has a thickness of 100 nm or greater. Furthermore, for example, if the composition wavelength of the SCH layer varies and the SCH layer absorbs the excitation light and generates carriers, these carriers also migrate to the laser portion multi-quantum well layer 204 and are observed as PL light. Therefore, variation in the composition wavelength of the SCH layer can cause variations in the intensity of the PL light. To avoid this, the influence of light absorption by the SCH layer can be reduced by ensuring that the light absorbed by the PL stabilization layer 209 is sufficiently greater than the light absorbed by the SCH layer. Therefore, it is preferable that the PL stabilization layer 209 be thicker than the n-type SCH layer 203. Furthermore, the thickness of the PL stabilization layer is preferably at least twice the total thickness of the layers with a composition wavelength capable of absorbing YAG laser light, excluding the laser portion multiple quantum well layer 204. Here, a layer with a composition wavelength capable of absorbing YAG laser light refers to a layer with a composition wavelength shorter than 1.064 μm. If the excitation light source is not a YAG laser, a layer with a composition wavelength capable of absorbing excitation light refers to a layer with a composition wavelength shorter than the wavelength of the excitation light source. In this embodiment, the layers that may absorb YAG laser light due to manufacturing variations are the n-type SCH layer 203 and the p-type SCH layer 205. The total thickness of these two layers is 50 nm. Meanwhile, the thickness of the PL stabilization layer 209 is 100 nm, which reduces the effect of composition wavelength variations in the SCH layer on PL light intensity. Therefore, this disclosure is particularly effective when the composition wavelength of the SCH layer is 1.03 μm or more and 1.09 μm or less.

Furthermore, to avoid the effects of variations in the composition wavelength of the completed PL stabilization layer 209, the composition wavelength of the PL stabilization layer 209 is preferably 1.1 μm or longer. The composition wavelength of the PL stabilization layer 209 is also preferably shorter than the composition wavelength of the well layer. If the composition wavelength of the PL stabilization layer 209 is longer than that of the well layer, the rate at which carriers recombine in the PL stabilization layer 209 before recombining in the laser portion multiple quantum well layer 204 increases, making it impossible to adequately supply carriers to the laser portion multiple quantum well layer 204. Furthermore, if the energy barrier between the adjacent n-type SCH layer 203 is too large, carriers generated in the PL stabilization layer 209 will have difficulty migrating toward the laser portion multiple quantum well layer 204. Therefore, it is preferable that the energy barrier between the PL stabilization layer 209 and adjacent layers be 0.23 μm or less in terms of composition wavelength. In this embodiment, the composition wavelength of the PL stabilization layer 209 is preferably 1.28 μm or less. Furthermore, the present disclosure is particularly effective when the composition wavelength of the PL stabilization layer 209 is longer than that of the n-type SCH layer 203. When the composition wavelength of the n-type SCH layer 203 is longer than that of the PL stabilization layer 209, the YAG laser light is absorbed by the n-type SCH layer 203, and the pump light is absorbed even without the PL stabilization layer 209, reducing the benefit of providing the PL stabilization layer 209. However, from the perspective of supplying more carriers to the laser portion multiple quantum well layer 204, there is an advantage to providing the PL stabilization layer 209.

Furthermore, the PL stabilization layer 209 is preferably an n-type layer or an undoped layer with a lower concentration than the substrate 202 and the n-type SCH layer 203. Here, "low concentration" refers to a carrier concentration of less than 1×10^17/cm3. An undoped layer refers to a layer that is not intentionally doped with impurities, including cases where impurities are present at background levels. If the PL stabilization layer 209 is an n-type layer with a high concentration, for example, a concentration of 1×10^17/ ^cm3 or higher, the effective energy gap increases. In other words, the effective absorption wavelength of the PL stabilization layer 209 shifts to shorter wavelengths. As a result, the composition becomes unable to absorb excitation light, potentially reducing the stability of PL measurements.

After verifying the quality of the semiconductor laser 211 through PL measurements using the above procedure, multilayer growth is performed on the EA modulator 213 using known lithography techniques, butt jointing, crystal growth, and other methods. When fabricating the EA modulator 213, the PL stabilization layer 209, similar to the semiconductor laser 211, allows for quality verification of the EA modulator's multiple quantum well layer. Similarly, the waveguide 212 is formed, followed by the p-type cladding layer 216 and p-type contact layer 218. Before forming the p-type cladding layer 216, a diffraction grating is formed in the p-type SCH layer 205 of the semiconductor laser 211. After multilayer growth, a mesa structure is formed, and BH layers 215 are formed on both sides of the mesa structure. Furthermore, an insulating film 219, a laser electrode 207, and a modulator electrode 225 are formed on the surface. After the substrate 202 is thinly polished, a back electrode 208 is formed on the back surface. Finally, chipping completes the semiconductor optical device 201.

The PL stabilization layer 209 has a smaller bandgap than the n-type SCH layer 203. Generally, the bandgap is gradually increased from the barrier layer of the multiple quantum well layer 4 toward the base substrate 202, thereby facilitating smooth carrier movement. Therefore, in the semiconductor optical device 201, the PL stabilization layer 209 may act as a barrier to carrier movement during actual operation. However, because the semiconductor laser 211 oscillates continuous light, the carrier movement speed is not a major concern. Therefore, even with the PL stabilization layer 209 according to the present disclosure, practical use is still possible. When the EA modulator 213 or this structure is applied to a directly modulated semiconductor laser, the modulation speed may be limited in some cases. In this case, the effect of reduced response speed can be suppressed by making the PL stabilization layer 209 as thin as possible and narrowing the bandgap with adjacent layers. Specifically, in the case of a semiconductor optical device compatible with the 1.3 μm band, it is preferable that the composition wavelength of the PL stabilization layer 209 be 1.1 μm or more, which allows reliable light absorption even when affected by manufacturing variations, and that the thickness be 300 nm or less. For other wavelength bands, it is preferable to use the thickness shown in the first embodiment.

Furthermore, it is preferable to place the PL stabilization layer 209 on the n-side of the multiple quantum well layer 4. If it is placed on the p-side, a large energy barrier will be generated because the PL stabilization layer 209 is undoped, which will have a significant impact on the characteristics of even a semiconductor laser. Note that the PL stabilization layer 209 is not limited to InGaAsP, as long as it has a composition wavelength that can absorb YAG laser light. For example, the PL stabilization layer 209 may be InGaAsAl.

[Third embodiment]
9 is a cross-sectional view of a semiconductor optical device 301 according to the third embodiment. The only difference from the first embodiment is that the positions of the PL stabilization layer 9 and the n-type SCH layer 3 are reversed.

In the third embodiment, the PL stabilization layer 9 is close to the multiple quantum well layer 4, allowing for a more stable supply of carriers during PL measurement. However, the optical confinement ratio of the multiple quantum well layer 4 is reduced compared to the first embodiment. A reduced optical confinement ratio has the advantage of reducing internal loss and enabling higher-power laser light to be emitted. A disadvantage is that the thick undoped layer (PL stabilization layer 9) is close to the multiple quantum well layer 4, reducing the electric field strength applied to the multiple quantum well layer 4. For example, if this structure is applied to an EA modulator rather than a semiconductor laser, the extinction ratio will be reduced.

[Fourth embodiment]
10 is a cross-sectional view of a semiconductor optical device 401 according to the fourth embodiment. The difference from the first embodiment is that the PL stabilization layer is divided into two layers. From the substrate 2 side, a first PL stabilization layer 409a, an n-type SCH layer 3, a second PL stabilization layer 409b, and a multiple quantum well layer 4 are stacked in this order. The first PL stabilization layer 409a and the second PL stabilization layer 409b have the same composition wavelength and are both undoped layers.

In this embodiment, the total thickness of the first PL stabilization layer 409a and the second PL stabilization layer 409b is the same as the PL stabilization layer 9 in the first embodiment. Therefore, as described in the first embodiment, it is possible to achieve stable PL measurements. Furthermore, because the PL stabilization layer is divided into two, it is possible to suppress carrier stagnation during modulation operation compared to when a single PL stabilization layer is provided. This embodiment is particularly effective in, for example, semiconductor lasers and EA modulators that perform modulation operation. Note that, while the PL stabilization layer is composed of two layers in this embodiment, it is not limited to this and may be divided into three layers. An SCH layer or other layer may be sandwiched between multiple PL stabilization layers. When multiple PL stabilization layers are provided, it is preferable that the composition wavelength of each PL stabilization layer be within the above-mentioned range and that they be undoped. Furthermore, it is preferable that the total thickness of the multiple PL stabilization layers be 100 nm to 300 nm.

As described above, by placing a PL stabilization layer between the multiple quantum well layer and the n-type semiconductor substrate, it is possible to stabilize the PL intensity during PL measurement, making it possible to accurately inspect crystal quality.

Furthermore, while an example has been shown in which only a PL stabilization layer and an SCH layer are disposed between the multiple quantum well layer 4 and the substrate 2, other layers may also be present. Furthermore, the ideas shown in the third and fourth embodiments may be combined with the second embodiment.

1, semiconductor optical element, 2 substrate, 3 n-type SCH layer, 4 multiple quantum well layer, 5 p-type SCH layer, 6 cladding layer, 7 front electrode, 8 back electrode, 9 PL stabilization layer, 10 cap layer, 201 semiconductor optical element, 202 substrate, 203 n-type SCH layer, 204 laser portion multiple quantum well layer, 205 p-type SCH layer, 207 laser portion electrode, 208 back electrode, 209 PL stabilization layer, 210 p-type InP cap layer, 211 semiconductor laser, 212 waveguide, 213 EA modulator, 215 BH layer, 216 p-type cladding layer, 218 p-type contact layer, 219 insulating film, 221 front end facet, 222 rear end facet, 225 modulator portion electrode, 301 semiconductor optical element, 401 semiconductor optical element, 409a First PL stabilization layer, 409b second PL stabilization layer.

Claims (14)

a substrate of a first conductivity type;
an optical confinement layer of a first conductivity type disposed above the substrate of the first conductivity type;
a multi-quantum well layer composed of a plurality of well layers and barrier layers disposed above the first conductivity type optical confinement layer;
an optical confinement layer of a second conductivity type disposed on the multiple quantum well layer;
a PL stabilization layer disposed between the first conductivity type substrate and the multiple quantum well layer;
the thickness of the PL stabilization layer is at least half the thickness of the multiple quantum well layer;
a composition wavelength of the PL stabilization layer is shorter than a composition wavelength of the well layer of the multiple quantum well layer and longer than a composition wavelength of the first conductivity type optical confinement layer;
the thickness of the PL stabilization layer is at least twice the total thickness of the first conductivity type light confinement layer and the second conductivity type light confinement layer;
Semiconductor optical element. 2. The semiconductor optical device according to claim 1,
A semiconductor optical device, wherein the PL stabilization layer has a carrier concentration that is undoped or less than 1×10^17/cm3. 3. The semiconductor optical device according to claim 1,
A semiconductor optical device, wherein the composition wavelength of the PL stabilization layer is longer than the wavelength of the excitation light used in measuring the PL of the multiple quantum well layer. 4. The semiconductor optical device according to claim 3 ,
a composition wavelength of the first conductivity type optical confinement layer and the second conductivity type optical confinement layer is longer than a wavelength of the excitation light; 5. The semiconductor optical device according to claim 1,
The PL stabilization layer is a multi-layer structure,
A semiconductor optical device, wherein the thickness of the PL stabilization layer is the total thickness of the plurality of layers. 6. The semiconductor optical device according to claim 1,
The PL stabilization layer is disposed between the substrate of the first conductivity type and the optical confinement layer of the first conductivity type. 7. The semiconductor optical device according to claim 1,
The PL stabilization layer is disposed between the multiple quantum well layer and the first conductivity type optical confinement layer. 8. The semiconductor optical device according to claim 1,
The first conductivity type is n-type, and the second conductivity type is p-type. 8. The semiconductor optical device according to claim 1,
The multiple quantum well layer emits and absorbs light in the 1.3 μm band,
the composition wavelength of the PL stabilization layer is 1.1 μm or more;
The PL stabilization layer has a thickness greater than that of the first conductivity type light confinement layer. 10. The semiconductor optical device according to claim 9,
The PL stabilization layer has a composition wavelength of 1.28 μm or less. 11. The semiconductor optical device according to claim 9,
a bandgap wavelength of the first conductivity type optical confinement layer and the second conductivity type optical confinement layer is 1.03 μm or more and 1.09 μm or less. 12. The semiconductor optical device according to claim 1,
The PL stabilization layer has a thickness of 100 nm or more and 300 nm or less. 6. The semiconductor optical device according to claim 5,
The semiconductor optical device, wherein the total thickness of the plurality of layers is 100 nm or more and 300 nm or less. 14. The semiconductor optical device according to claim 1,
The semiconductor optical element is a semiconductor laser.